1. Field of the Invention
This invention relates generally to devices for measuring and recording the pressure in oil and gas wells, specifically to a device which, when connected to a mechanical recorder of the type now commonly in use, will allow the pressure measurements to be converted into corresponding voltage changes and stored in a memory device. More particularly, this invention relates to a downhole device meant to be lowered into the well bore to take pressure measurements, as opposed to devices used at the surface to measure well pressure.
2. Description of the Prior Art
There are currently two basic types of downhole pressure gauges commonly available to measure and record pressure information about oil and gas wells. One of these types, the older one, is a mechanical device, while the newer one uses electronics.
The downhole mechanical pressure gauges have been around for over sixty years. When completely assembled, they are in the form of a long tube with a bullet shape on one end, and an eyelet or socket on the other which allows a wire to be passed through and attached. This wire is then used to lower the pressure gauge into the well bore and retrieve it after the measurement has been completed.
Typically, the fully-assembled mechanical gauge actually consists of three separate parts which have been threadedly attached together to form an elongated tube. The first part contains a Bourdon pressure sensing element attached to a connecting pin. As the Bourdon tube detects changes in well pressure, these pressure changes are translated into a corresponding rotational movement in the connecting pin. This part of the pressure gauge, with the Bourdon tube, is connected to the second part of the pressure gauge, which contains a recording unit and clock. The second part is threaded together with the first part in such a manner that the connecting pin engages a stylus when the two parts are joined. The stylus, in turn, makes contact with a recording chart, enclosed in a chart holder. The recording chart is a thin metal sheet rolled into a cylinder shape and inserted into the chart holder. The chart holder, in turn, is attached to a mechanical clock which drives the chart holder forward, relative to the stylus, at a constant speed for a pre-determined amount of time as chosen by the user. As the well pressure fluctuates over time, the changes are detected by the Bourdon tube and translated into rotation in the connecting pin and the stylus. As the stylus moves, it scratches recording the chart, thereby recording fluctuations in pressure. Meanwhile, the mechanical clock is driving the chart holder forward at a constant rate. The net result is the production of a chart which shows well pressure as a function of time. The third part of the mechanical pressure gauge threadedly attaches to the second part at the end opposite the end attached to the first part and provides a means for the attachment of the wire used to raise and lower the mechanical pressure gauge.
In a typical application, a new chart will be inserted into the chart holder, the mechanical clock will be set, and the fully assembled pressure gauge will be lowered into the well. At the end of the test period, the pressure gauge is retrieved, and the chart removed and read to obtain a record of the well's pressure over the test period.
The second basic type of downhole pressure gauge commonly in use in the oil and gas industry is an electronic memory pressure recorder. These devices have been around since the late 1970's. Instead of a Bourdon tube to detect pressure changes, they use various electronic means for the same purpose. For example, some use the piezometric properties of quartz crystals to produce a voltage signal that varies with pressure, and which can be converted into meaningful pressure data. Other gauges of this type use strain gauge transducers to produce the voltage changes. Regardless of which means is used to produce the pressure-sensitive voltage changes, the resulting information is sent to an electronic memory storage board. The pressure sensitive voltage device and electronic memory storage board are placed into a housing of a size and shape convenient for lowering into the well, such as an elongated tube similar to the mechanical pressure gauges.
In a typical application, the electronic pressure gauge will be lowered into a well for the test period and then retrieved. The information which has been stored in the memory board can then be downloaded into a suitable device where the time/pressure information can be converted into a convenient form. Alternatively, the electronic pressure gauge is also capable of allowing well pressure to be monitored at the surface while the test is ongoing if an electrical line carrying the voltage changes is attached to the unit and run to the surface during testing, thereby allowing remote, instantaneous access to detected pressure changes.
Both of the basic types of downhole pressure gauge described above have advantages and disadvantages. The advantages of the mechanical pressure gauge are its low cost, availability, and durability. The chief disadvantages are the relatively poor accuracy of its pressure measurements, especially as the time of testing increases, and the inability to use it to conduct remote testing. The advantages of the electronic pressure gauge are its high accuracy, ease of data retrieval, and remote testing capabilities. The chief disadvantage of the electronic pressure gauge unit is its high cost, which simply cannot be justified or afforded by many small oil and gas operators.